Ram air turbine



Dec 6, 1960 Filed Oct. 7, 1955 C. F. DREXEL ETAL RAM AIR TURBINE 3Sheets-Sheet 1 CHARLES F." DREXEL,

CASS/US E. SEWELL,

JAMES E. YOU/V6,

INVENTORS.

qhm z mz Dec. 6, 1960 c. F. DREXEL ETAL RAM AIR TURBINE Filed 001:. 7,1955 3 Sheets-Sheet 2 CHARLES E DREXEL, CASS/U5 E. .SEWELL,

JAMES E. YOUNG INVEA/TORS.

Dec. 6, 1960 c. F. DREXEL ETAL 2,963,093

' RAM AIR TURBINE Filed Oct. 7, 1955 3 Sheets-Sheet 3 CHARLES E one-x11,cAss/us E. saws, JAMES E. YOU/V6,

. INVENTORS.

BY 7 M v 81121138 patgnt RAM AIR TURBINE Charles F. Drexel, Palos VerdesEstates, James E. Young, North Hollywood, and Cassius E. Sewell,Gardena, Califi, assignors to The Garrett Corporation, Los Angeles,Calif., a corporation of California Filed Oct. 7, 1955, Ser. No. 547,582

7 Claims. (CL 17068) ,This invention relates to a ram air driven turbineand, more particularly, to improvements in turbines of the gen- 'e'raltype disclosed in Patent No. 2,777,524, dated January 15, 1957, to JamesE. Chapman, Charles F. Drexel, and James .E. Young.

' Ram air driven turbines of prior designs have been found unsuitablefor driving alternators because they were incapable of operating withinsufiiciently close speed tolerances to insure a precise power output. Ithas been proposed to provide such turbines with centrifugally foperatedmeans to adjust the pitch of the turbine blades for-controlling thespeed of the turbine, but this expedient in prior designs has been foundto be unsatisfactory dueto the fact that the centrifugal forces of therapidly moving blades impose such frictional loads on the blade bearingsthat the blades cannot be adjusted as quickly or as accurately as isnecessary to secure the desired results.

It has also been found that the inability of prior centrifugallyoperated mechanisms to control the speed of ram 'air driven turbineswithin the fine limits required, has been :due, in part, to changingforces or loads imposed on the mechanism by aerodynamic disturbancesoccasioned by variations in the angles of attack of the turbine blades.It is an object of this invention to provide a ram air driven turbinehaving an improved centrifugally operated means for adjusting the bladeswhich will avoid the objections of the prior devices by respondinginstantly and accurately to changes in rates of rotation of the turbine.

Another object of the invention is to provide a ram 'air'driven turbinehaving means for balancing the centrifiigall forces developed in theblades and thus eliminate the 'excessive loading of the bearings and theobjections incident thereto. 1' A further object of the invention is toprovide a ram air driven turbine having blades with shanks or tie barswhich extend through the axis of rotation of the turbine and which areprovided, on the opposite side of the axis from the blades, withcounterweights which develop centrifugal forces to offset or counteractthe similar forces developed in the blades during high speed operationof the turbine.

A still further object of the invention is to provide a ram air driventurbine having a plurality of blades extending radially from the axis ofrotation of a main shaft, the blades having shanks spaced longitudinallyof the main shaft and supported for rotary adjustment on opposite sidesof the main shaft in antifriction bearings, the shanks being provided onthe opposite side. of the axis from the blades with co-unterweights toset up centrifugal forces when the turbine operates at high speed, suchforces opposing those set up by the blades and thus relieving theantifriction bearings from excessive thrust.

A still' further object of the invention is to provide a ram air driventurbine of the type set forth in the preeeglingparagraph with means forplacing the blade shanks nn'dertension to initially apply a thrust loadto 'the,bearings'in afdirection' opposed to that applied by thecentriftigal' force, so that when the turbine is operated at high speedthe centrifugal forces of the blades and counterweights will relieve theinitially applied thrust load and the bearings will operate under littleor no load and substantially without friction.

Another object of the invention is to provide a ram air driven turbinewith blades designed to avoid the transmission of undesirable torque tothe blade shafts and associated mechanisms, regardless of the angles ofattack of the blades, such construction improving the operation of thedevice by eliminating any tendency of the automatic blade-adjustingmechanism to hesitate, lag or be sluggish in operation when the anglesof attack of the blades are to be changed because of a variation in airspeed. I I

Another object of the invention is to provide a ram air driven turbinewith blades having an airfoil configuration which disposes the center ofpressure of each blade at a predetermined location, and to arrange theblades on the turbine for adjustment about axes passing through thecenters of pressure so that the application of undesirable aerodynamicforces to the blades when the pitch of the blades is changed, will beeliminated, special counterweight elements being provided to offsetcentrifugal forces incident to the particular relation of the blades andpitch adjustment axes.

Still another object of the invention is to provide a ram air driventurbine having a main shaft and a plurality of blades extending radiallyfrom the axis of rotation of such shaft, the blades being disposed foradjustment in planes spaced longitudinally of the axis of the mainshaft, the turbine having means responsive to centrifugal forces foradjusting the blades, and having novel motion transmitting means betweenthe centrifugal force creating means and the blades, to cause asimultaneous adjustment of the latter. I

Other objects and advantages of the invention may be apparent from thefollowing specification, appended claims, and accompanying drawings inwhich:

Figure l is a vertical longitudinal sectional view of a ram air turbineformed in accordance with the present invention, parts being broken awayand other portions being shown in elevation to facilitate theillustration;

Figure 2 is a vertical transverse sectional view taken on planesindicated by the staggered line 2--2 of Figure 1;

Figure 3 is a similar view taken on planes indicated by staggered line33 of Figure 1',

Figure 4 is a detailed sectional view on an enlarged scale taken on theplane indicated by the line 44 of Figure 3;

Figure 5 is a perspective view of one of the ram air turbine bladesshowing a portion of a shaft connected therewith;

Figure 6 is a vertical transverse sectional view taken on the planeindicated by the line 66 of Figure 1;

Figure 7 is a fragmentary sectional view taken on the plane indicated byline 7-7 of Figure 2;

Figure 8 is a perspective view of a spider gear employed to actuate theturbine blades about their pitch adjustment axes; and p Figure 9 is adiagrammatic view illustrating the crosssectional shape of the presentram air turbine blades.

The terms angle of attac and pitch as used herein are substantiallysynonymous. However, the term angle of attack is employed principally inreference to the aerodynamic disposition of the turbine blades, whilethe term pitc is employed in reference to the mechanical position or theangular disposition of the chord of a selected blade with reference to'a plane parallel to the longitudinalaxes of the turbine and such blade.v v Asshown in Figure 1 of the drawings, a ram air turbinefaccording tothe present invention, is provided with a stationary housing 10 havingan aerodynamic strut 11 which se'rves as a support for the housing 10when the ram air turbine is disposed in projected operating position.The housing is provided with an end cap 12 in which a bearing 13 issupported. This bearing 13 carries one end of a hollow rotor shaft 14which is supported at its opposite end in a bearing 15 mounted in thehousing 10. A shaft 16 extends through the hollow rotor shaft 14 and isengaged therewith by means of splines 17. The ends of shafts 14 and 16are secured in axially fixed relation with the bearing 13 by means of alocknut 18. The shaft 14 carries a rotor structure 19 which co-operateswith a field structure 20, the rotor and field structures comprising analternator or generator for producing electrical power.

Shaft 16 is provided with an enlarged portion 21 journaled in a bearing22 which is supported in an extension 23 of the housing 10. Integralwith the shaft 16 is a flange 24 which supports the rotating structureof the ram air turbine. it Will be noted that the axis of the shaft 16is common to the rotating structure of the generator and the ram airturbine. The rotating structure of the ram air turbine comprises aspinner housing member 25 which is connected to the shaft flange 24 andwhich surrounds the extension 23 of the housing 10. Secured to the rearportion of the spinner housing member 25 by means of bolts 26, seeFigure 6, is a plate 27 which forms the rear end of the turbine spinnerhousing. Fixed to the spinner housing member 25 by means of bolts 28 isthe spinner housing member 29. This housing member 29 serves as asupport or carrier for turbine blades 30, 39a, and 36b which projectradially of the housing 29 and pivot about their axes at substantiallyright angles to the common axis of the shaft 16 and the spinner housingmembers 25 and 29. Since the turbine blades 30, 30a, and 30b and theirconnected centrifugal balancing mechanism is similar, the structuralelements of the turbine blade 30 only are hereinafter described indetail.

As shown in Figure l of the drawing, the turbine blade 30 extendsoutwardly of the spinner housing members 25 and 29 and operates in theair stream which passes therearound. The blade 39 is provided with a hub31 supported in a bearing 32 which is carried in a recess 33 in thehousing member 29. The blade 30 is provided with gear teeth 34 whichmesh with conforming teeth 35 of a blade synchronizing spider gear 36.Threadedly connected to the hub 31 is a shaft 37 having an enlargedopposite end 38 supported in a bearing 39 carried in a recess 40 in thehousing member 29. Threaded onto the shaft 37 is a nut 41 which fixes acounterweight 42 against a shoulder 43 on the enlarged portion 38 ofsaid shaft 37. The counterweight 42 is arranged to counteract andprevent torsional forces in the blade 30 resulting from centrifugalaction, from affecting the operation of the pitch adjusting mechanism,as will be hereinafter described in detail.

A screw 44 is threaded into a conforming threaded hole in the endportion 38 of the shaft 37 and this screw 44 is provided with a largehead 45 arranged to engage the inner race of the bearing 39 and retainthe shaft 37 in a certain axial position relative to the bearings 32 and39. The hub 31 of blade 30 is provided with a shoulder 31a to engage theinner race of the bearing 32 and when screw 44 is engaged with the innerrace of bearing 39 and tightened, the shaft or tie rod 37 is stressed toapply an inward thrust or preload on the bearings 32 and 39. The blade30, hub 31, and portions of the bearing 32 which are connected thereto,together with a portion of the shaft 37 extending on one side of theturbine axis, are designed to have a centrifugal force moment equal tothat of the structure connected to the shaft 37 and ex.- tendingradially of said axis in the opposite direction. Thus, the mass of theparts of each blade assembly on one side of the axis of the turbinebalances the mass of the parts of the assembly on the opposite side ofthe axis. When the turbine is operated, the centrifugal forces of theparts of each blade assembly on each side of the tur- 4 bine axis willbe ofiset or balanced by those of the parts on the other side of theaxis and the turbine will operate smoothly. The centrifugal forces ofthe parts of the blade assemblies will also tend to stretch the shaftsor tie rods 37 and relieve the bearings 32 and 39 of the preloadinitially applied through the tightening of the screw 44. The hearingswill, therefore, be subject to a minimum thrust load and will offer onlynormal resistance to rotation at the designed operating speeds of theturbine.

Figure 9 shows a typical cross section of the turbine blades. From thisfigure it is apparent that each blade is symmetrical about the meanchord 30c of such blade. This blade configuration provides a center ofpressure 30d which, as shown in Figure 9, is located approximately 27%(between 25 and 30%) of the chord length from the leading edge 30a ofthe blade, and the latter is so located on the hub 31 that this centerof pressure is disposed on the axis of pivotal movement of the blade inthe bearings 32 and 39. Due to this arrangement, the relationship of thecenter of pressure to the axis of movement of the blade will remainconstant even during pitch adjustment of the blade; this design,therefore, is particularly adaptable to spring biased, centrifugallyoperated, pitch adjusting mechanism, a form of which is employed in thepresent air driven turbine.

The location of the axis of pitch adjustment of each blade in the mannerjust described places the center of gravity of the blade at one side ofthe adjustment axis and when the turbine revolves, centrifugal forcesmay transmit undesirable torque to the blade. This effect iscounteracted in the form of the invention illustrated, by providing thecounterweight 42 with a section 420 which, as shown in Figure 4, islocated on the opposite side of the axis of adjustment of the blade fromthe blade, per se. It should be obvious that the shape of the weight 42may vary considerably from that shown, the important feature being thelocation of the center of gravity thereof so that the torque transmittedto the blade by centrifugal forces acting thereon will be counteractedby similar forces of the counterweight.

As shown in Figure 3 of the drawings, the turbine blades 30a and 30b areprovided with gear portions 46 and 47, respectively, which are similarto the gear 34 of the turbine blade 30. The gear elements 34, 46, and 47mesh with gear portions 35, 49, and 50 of the spider gear 36. The gearportions 35, 49, and 50 are axially spaced relative to each other inorder to accommodate a staggered relationship of the shafts 37 of theturbine blades 30, 30a and 30b. This spider gear 36 is provided with ahollow shaft portion 51 journaled in bearings 52 and 53 carried by thespinner housing member 29. Fixed to the spider gear 36 by means of aloclcnut 54 and spline 54a is a shaft 55 which extends through thehollow shaft 51 and carries a gear 56 in meshing engagement with gearsegments 57, 58, and 59 shown particularly in Figure 2 of the drawings.

Each of the gear segments 57, 58 and 59 is connected to a torsion barand flyweight structure. In the present ram air turbine, there are threetorsion bars and three flyweights, all of which are similar inconstruction. Therefore, a torsion bar 60, together with its connectedflyweight 61, is herein described and shown in detail in Figure 7 of thedrawngs. The gear segment 58 is fixed to a tubular member 62 journaledin bearings 63 and 64 which are supported in the spinner housing member29. Fixed on the outer side of the tubular member 62, adjacent the gear58, is the flyweight 61. (inc end 65 of the torsion bar is fixed in thetubular member 62 and the opposite end of the torsion bar 60 is fixed ina tubular member 66 which is rotatably mounted in the spinner housingmember 25 and end plate 27.

Referring to Figure 6 of the drawings, it will be seen that the tubularmember 66 is provided with a projecting arm 67 which is engaged with anadjusting screw 68 in threaded in the spinner housing member 25a'nd'arranged .to provide an adjustable stop for the torsion bar 60. Asecond adjustable stop means in the form of screws 70 is provided, asshown in Figure 2, for engagement by the fiyweights at the forward endsof the torsion bars. When the screws 68 are adjusted inwardly, the arms67 turn the torsion bars and tend to swing the flyweights 61 intoengagement with the screws 70. This engagement precludes furthermovement of the flyweight; continued adjustment of the screws 68inwardly preloads the torsion bars 60. The starting or initial pitch ofthe blades may be varied through the adjustment of the stop screws 70.

Operation When the present ram air turbine is supported in an 5 airstream, .theblades 30, 30a and 30b tend to rotate the shaft 16 andspinner housing members 25 and 29 about the axis of the bearings 13, 15,and 22. As the rotating "mechanism of the turbine reaches a desiredpredetermined speed, any tendency toward exceeding such speed isautomatically opposed byra change in pitch of the blades through themovement of the fiyweights in opposition to the resistance of thetorsionbars. When the fiyweights swing outwardly under centrifugal force, thegears 57, 58, and 59 transmit rotary motion through gears 56 and 36 tothe gear elements 34, 46, and 47 connected with the blades 30, a, and30b, respectively, and the pitch of the blades will be changed. Rotarymovement of the flyweights imparts torsional stress to Qthebars 60 andwhen the speed of operation of the tur- 30 bine decreases, the forcesstored in the bars 60 will tend to return the blades and associatedmechanism to .the initial starting position; the design speed of theturbine .will thus be maintained at dilferent air stream speeds. 3O .The.counterweights 42 connected with the turbine blades are arranged toprevent centrifugal torsion forces, acting. on the blades, fromaffecting the balanced operation of the torsion bars relative to thecentrifugal flyweights 61 connected thereto. 4 It is recognized that theforce balance between the centrifugally operated flyweights and theresilient tor- :sion bars may be very delicate with respect to themainte- -nance of speed variations of the turbine within closetolerances. Therefore, it is desired to reduce rotational 5 resistancein the present turbine blade bearings to a minimum. Centrifugal forcesacting axially on the turbine blades are balanced by similar forcesacting on mechanism diametrically opposed to the blades whereby theblade supporting bearings operate with a minimum 50 of rotationalresistance. Due to the fact that the rotational speed of the present ramair turbine is precisely maintained, it may be used to operate analternator or any other device requiring operation at a speed to bemaintained within critical limits. i

- We claim: 1. In a ram air turbine having a central axis of'rotation: ablade carrier supported coaxially with and disposed for rotation aboutsaid central axis of rotation; a pair of bearing means on said carrierat diametrically opposite sides of said central axis of rotationrespectively; an assembly including a blade having a shaftmeansextending from one end of said blade and counterweight {means on saidshaft means, said shaft means being rotat- {ably supported by saidbearing means with the axis thereof intersecting and disposedsubstantially at right-angles to said central axis of rotation, saidcounterweightmeans being disposed on the opposite side of said centralaxis of rotation from said blade, each of said pair of bearing meanscooperating with the portion of said shaft supportedthereby to resistaxial shifting of such portion ,toward said central axis of rotation;means cooperating with at least one of said pair of bearing meansinitially loading said pair of bearing means to place the section ofsaidshaft means between said pair of bearing means under tension, theportion of said assembly and the portion of the bearing means rotatablewith said shaft means on one side of said central axis of rotationsubstantially counterbalancing the portion of said assembly andtheportion of the bearing means rotatable with said shaft means on theopposite side of said central axis of rotation whereby substantiallyequal and opposite centrifugal forces are applied to relieve the initialload on said pair of bearing means upon rotation of said carrier at apre.- determined speed.

2. In a ram air turbine having a central axis of rotation: a bladecarrier supported coaxially with and disposed for rotation about saidcentral axis of rotation; a pair of bearing means on said carrier atdiametrically opposite sides of said central axis of rotation respec-,tively; an assembly including a blade' having a shaft means extendingfrom one end of said blade and counterweight means on said shaft means,said shaft means being rotatably supported by said bearing means withthe axis thereof intersecting and disposed substantially at right anglesto said central axis of rotation, said counterweight means beingdisposed on the opposite side of said central axis of rotation from saidblade, each of said pair .of bearing means cooperating with the portionof said shaft supported thereby to resist: axial shifting of suchportion toward said central axis of rotation; resilient means tending toresist movement of said'shaft means about itsfaxis; fiyweight meansresponsive torotation of -said ,bladefcarrier about said central axis ofrotation to move said shaft means about its axis in opposition to saidresilient means; 'means cooperating with at least one:of said 'painofbearin'g means initially loading said pair ofbear-ingrneansto placethe section of said shaft means between said-pair of bearing means undertension, the portion of said assembly and the portion of the bearingmeans rotatable with said shaft means onone side of said central axis ofrotation substantially counterbalancing the portion-10f said assemblyand the portion of the bearing means rotat- Q able with said shaft meanson the opposite side ofsaid central axis of rotation wherebysubstantially equal and opposite centrifugal forces are applied to"relieve the initial load on said pair of bearing means upon rotation ofsaid carrier at a predetermined speed. 1

3. In a ram air turbine having a central axis of'rotation: a bladecarrier supported 'coaxially -withan'd'disposed for rotation about said:centraljaxis of; rotation; a plurality of pairs of bearing'means' onsaid carrier, each of said pairs being spaced axially andcircumferentially on said carrier, the bearing means. of eachfpair beingdisposed at-diametrically opposite sides of said centr'al axis ofrotation; an assembly for each of said pairs .of bearing means, saidassembly includingv a bladehav; ing shaft means extending from one endof said blade and also including counterweight means on saidshaft means,the. shaft means 'of each assembly being ,rotatably supported by a pairof said bearing means with the axis thereof intersecting and'disposedsubstantially at right angles to said central axis of 'rotation,',saidcounterweight means and said blade of each assemblybeingdisposed'on opposite'sides of said central axis'of, rotation, thebearing means'of each pair on each side of said central axis; ofrotation cooperating witl'rthe portion of the ;shaft means supportedthereby to resist axial shifting ofysuch portion toward said centralaxis of rotation; ;rneanscooperating with at least one of each pair ofbearing means t9,init i ally loa'd'said' pairs of bearing means to placethe section-of the shaft meansbetween the bearing means of-each pair.under tension, the portion of each said assembly andgthe portion of thebearing means rotatable with the -'shaft means of such assembly on oneside of said central axis 'of rotation substantially counterbalancing'ithe :portion of such assembly and the portion of the bearing niea'hsrotatable with the shaft means-of such assenilil y errata-e oppositeside of said central axis of rotation whereby substantially equal andopposite centrifugal forces are applied to relieve the initial load oneach said pair of bearing means upon rotation of said carrier at apredetermined speed.

4. In a ram air turbine having a central axis of rotation: a bladecarrier supported coaxially with and disposed for rotation about saidcentral axis of rotation; a plurality of pairs of bearing means on saidcarrier, each of said pairs being spaced axially and circumferentiallyon said carrier, the bearings means of each pair being disposed atdiametrically opposite sides of said central axis of rotation; anassembly for each of said pairs of bearing means, said assemblyincluding a blade having shaft means extending from one end of saidblade and also including counterweight means on said shaft means, theshaft means of each assembly being rotatably supported by a pair of saidbearing means with the axis thereof intersecting and disposedsubstantially at right angles to said central axis of rotation, saidcounterweight means and said blade of each assembly being disposed onopposite sides of said central axis of rotation, the bearing means ofeach pair on each side of said central axis of rotation cooperating withthe portion of the shaft means supported thereby to resist axialshifting of such portion toward said central axis of rotation; meansconmeeting the shaft means for synchronized adjustment of said bladesabout the axes of said shaft means; flyweight means responsive to therotation of said carrier about said central axis of rotation to movesaid shaft means to change the pitch of said blades; resilient means forresisting movement of said shaft means by said flyw'eight means; meanscooperating with at least one of each pair of bearing means to initiallyload said pairs of bearing means to place the section of the shaft meansbetween the bearing means of each pair under tension, the portion ofeach said assembly and the portion of the bearing means rotatable withthe shaft means of such assembly on one side of said central axis ofrotation substantially counter-balancing the portion of such assemblyand the portion of the bearing means rotatable with r the shaft means ofsuch assembly on the opposite side of said central axis of rotationwhereby substantially equal and opposite centrifugal forces are appliedto relieve the initial load on each said pair of bearing means uponrotation of said carrier at a predetermined speed.

In a ram air turbine as recited in claim 4 wherein each counterweightmeans is disposed relative to its associated shaft means and blade toresist centrifugal torsion forces developed about the axis of suchassociated shaft means and blade.

6. In a ram air turbine having a central axis of rotation: a bladecarrier supported coaxially with and disposed for rotation about saidcentral axis of rotation; a plurality of pairs of bearing means on saidcarrier, each of said pairs being spaced axially and circumferentiallyon said carrier, the bearing means of each pair being disposed atdiametrically opposite sides of said central axis'of rotation; anassembly for each of said pairs of bearing means, said assemblyincluding a blade having shaft means extending from one end of saidblade and also including counterweight means on said shaft means, theshaft means of each assembly being rotatably supported by a pair ofsaidbearing means with the axis thereof intersecting anddisposedsubstantially at right angles to said central axis of rotation,said counterweight means and said blade of each assembly being disposedon opposite sides of said central axis of rotation, the bearing means ofeach pair on each side of said central axis of rotation cooperating withthe portion of the shaft means supported thereby to resist axialshifting of such portion toward said central axis of rotation;centrifugally operated means to adiust the pitch of the blades inresponse to rotation of said blade carrier about said central axis ofrotation, each of the blades having an aerodynamically balancedconfiguration and being located with the center of pressuresubstantially coincidental with its pitch adjustment axis; meanscooperating with at least one of each pair of bearing means to initiallyload said pairs of bearing means to place the section of the shaft meansbetween the bearing means of each pair under tension, the portion ofeach said assembly and the portion of the bearing means rotatable withthe shaft means of such assembly on one side of said central axis ofrotation substantially counterbalancing the portion of such assembly andthe portion of the bearing means rotatable with the shaft means of suchassembly on the opposite side of said central axis of rotation wherebysubstantially equal and opposite centrifugal forces are applied torelieve the initial load on each said pair of bearing means uponrotation of said carrier at a predetermined speed.

7. In a ram air turbine having a central axis of rotation: a bladecarrier supported coaxially with and disposed for rotation about saidcentral axis of rotation; a plurality of pairs of bearing means on saidcarrier, each of said pairs being spaced axially and circumferentiallyon said carrier, the bearing means of each pair being disposed atdiametrically opposite sides of said central axis of rotation; anassembly for each of said pairs of bearing means, said assemblyincluding a blade having shaft means extending from one end of saidblade and also including counterweight means on said shaft means, theshaft means of each assembly being rotatably supported by a pair of saidbearing means with the axis thereof intersecting and disposedsubstantially at right angles to said central axis of rotation, saidcounterweight means and said blade of each assembly being disposed onopposite sides of said central axis of rotation, the bearing means ofeach pair on each side of said central axis of rotation cooperating withthe portion of the shaft means supported thereby to resist axialshifting of such portion toward said central axis of rotation;centrifugally operated means to adjust the pitch of the blades inresponse to rotation of said blade carrier about said central axis ofrotation, each of the blades being symmetrical about a mean chord andlocated with the center of pressure coinciding with said pitchadjustment axis to preclude the production of torque about such axiswhen the pitch of the blade is changed; means cooperating with at leastone of each pair of bearing means to initially load said pairs ofbearing means to place the section of the shaft means between thebearing means of each pair under tension, the portion of each saidassembly and the portion of the bearing means rotatable with the shaftmeans of such assembly on one side of said central axis of rotationsubstantially counterbalaucing the portion of such assembly and theportion of the bearing means rotatable with the shaft means of suchassembly on the opposite side of said central axis of rotation wherebysubstantially equal and opposite centrifugal forces are applied torelieve the initial load on each said pair of bearing means uponrotation of said carrier at a predetermined speed.

References Cited in the file of this patent UNITED STATES PATENTS1,636,434 Pinaud July 19, 1927 1,886,891 Martens et al. Nov. 8, 19321,887,543 Chilton Nov. 15, 1932 1,919,586 Dodge July 25, 1933 2,345,047Houghton Mar. 28, 1944 2,514,525 Stulen July 11, 1950 2.754.915Echeverria July 17, 1956 2,777,524 Chapman et a1. Jan. 15, 1957 FOREIGNPATENTS 510,307 Belgium Apr. 15, 1952

